The present invention relates to silver salt black-and-white photothermographic imaging material (hereinafter, also simply denoted as a photothermographic material) with minimal unevenness in density and exhibiting enhanced sensitivity and superior silver image tone.
In the field of graphic arts and medical treatment, there have been concerns in processing of photographic film with respect to effluent produced from wet-processing of image forming materials, and recently, reduction of the processing effluent is strongly demanded in terms of environmental protection and space savings. There has been desired a photothermographic dry imaging material for photographic use, capable of forming distinct black images exhibiting high sharpness, enabling efficient exposure by means of a laser imager or a laser image setter.
Known as such a technique are thermally developable silver salt photographic materials (which are the same as photothermographic materials, as described in the present invention) comprising on a support an organic silver salt, light-sensitive silver halide and a reducing agent, as described in D. Morgan and B. Shely, U.S. Pat. Nos. 3,152,904 and 3,487,075, and D. H. Klosterboer, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d in IMAGING PROCESSES and MATERIALS, Neblette""s Eighth Edition, edited by J. M. Sturge, V. Walworth, and A. Shepp (1969) page 279. The thermally developable silver salt photographic material provides a simply and environment-friendly system for users, without use of any processing solution.
These silver salt photothermographic materials comprise a light sensitive layer containing light sensitive silver halide grains as a photosensor and an organic silver salt as a silver ion source, which are thermally developed with a reducing agent at a temperature of 80 to 140xc2x0 C. to form images, without being subjected to fixing. To achieve smooth supply of silver ions and minimize lowered transparency due to light scattering, much effort has been made to improve the shape of organic silver salt particles so as to appropriately arrange the salt in the light sensitive layer without adversely affecting light scattering.
However, in silver salt photothermographic materials, inclusion of an organic silver salt, silver halide grains and a reducing agent easily results in fogging during the storage time prior to thermal development. Furthermore, there are problems that the photothermographic materials, after exposure are usually developed at a temperature of 80 to 250xc2x0 C. without being fixed, and the silver halide, organic silver salt and reducing agent concurrently remain in the layer, so that metallic silver is thermally or photolytically produced after storage over a long period of time, deteriorating image quality, such as silver image tone.
There were disclosed techniques for solving such problems in JP-A Nos. 6-208192 and 8-267934 (hereinafter, the term, JP-A refers to an unexamined, published Japanese Patent Application); U.S. Pat. No. 5,714,311 and references cited therein. These disclosed techniques had desired effects to some extents but were not sufficient as a technique to satisfy the level required in the market. In addition thereto, further enhanced image quality has been desired as a perpetual theme in silver salt photothermographic imaging materials. Specifically in the field of medical diagnostic imaging, further enhanced image quality is desired to effectuate more precise diagnosis.
In light of the foregoing, the present invention was accomplished. Thus, it is an object of the invention to provide a silver salt photothermographic imaging material exhibiting high image quality without causing unevenness in density and still maintaining superior silver image tone even after storage over a long period of time after having been subjected to thermal development, and an image recording method by the use thereof.
The foregoing object of the invention can be accomplished by the following constitution:
1. a silver salt photothermographic material comprising a support having provided thereon a light-sensitive layer containing light-sensitive silver halide, a light-insensitive organic silver salt and a binder, wherein the photothermographic material exhibits a gradation of 2.0 to 5.0 within the density region of 0.5 and 2.0 on a photographic characteristic curve obtained when the photothermographic material is subjected to exposure to light of 50 xcexcJ/cm2 and thermal development at 124xc2x0 C. for 16 sec.; the light-sensitive layer comprises at least two layers including a lower layer and a upper layer provided on the lower layer and a glass transition temperature of a binder contained in the lower layer is higher than that of a binder contained in the upper layer.
Further, preferred embodiments of the invention are as follows:
2. the photothermographic material described in 1. above, wherein a mean radius of spheres of influence of the lower layer (also denoted as r) than the upper layer is less than that of the upper layer (also denoted as rxe2x80x2);
3. the photothermographic material described in 1. or 2. above, wherein the mean radius of spheres of influence of the upper layer (rxe2x80x2) is 0.30 to 1.10 xcexcm;
4. the photothermographic material described in 1., 2. or 3., wherein the light-sensitive layer contains a silver halide emulsion which has been sensitized with a chalcogen sensitizer; and
5. the photothermographic material described in any of 1. through 4. above, wherein the light-sensitive layer or light-insensitive layer contains a silver-saving agent.